This invention relates to a method and an apparatus for imaging a section of a human body by means of nuclear magnetic resonance phenomena, and more specifically to a method and an apparatus for high resolution nuclear magnetic resonance imaging suitable for medical diagnoses and/or treatments.
A signal detected by an NMR imaging apparatus is called an NMR signal which is produced when a nuclear spin returns from an energy level in a stage thereof excited by a resonance signal to that in its ground state.
The detected signal can be represented by the following equation: ##EQU1## where M.sub.0 (x): magnetic field intensity at time 0
G: gradient magnetic field PA0 T.sub.2 (x): transverse relaxation time PA0 K: proportionality constant PA0 .gamma.: nuclear magnetic rotation ratio
As it can be seen from equation (1), the detected signal decreases exponentially with a time constant of the transverse relaxation time T.sub.2 (x) (hereinbelow abbreviated simply to T.sub.2). T.sub.2 is usually of an order of magnitude of 10.sup.-5 .about.1 second. However, by influences of inhomogeneity of the magnetic field the actual decrease time constant T.sub.2 * is changed as follows: ##EQU2## where .DELTA.H is inhomogeneity of magnetic field. That is, since T.sub.2 *.ltoreq.T.sub.2, the time constant becomes further smaller and the detected signal decreases more rapidly.
On the other hand, the resolving power for images is determined by a sampling interval .DELTA..omega. after the Fourier transformation of the NMR signal. Representing a measureing time by T, the following relationship is satisfied: EQU .DELTA..omega.=2.pi./T (3)
Further, for an NMR imaging apparatus, the gradient magnetic field G, a spatial sampling width .DELTA.Z and the frequency sampling interval .DELTA..omega. satisfy the following relationship: EQU .DELTA..omega.=.gamma.G.DELTA.z
Then, .DELTA.z can be expressed as follows: EQU .DELTA.z=2.pi./.gamma.TG (4)
That is, .DELTA.z becomes smaller and the spatial resolving power increases with increase in the gradient magnetic field G and increase in the measuring time T.
However, when the gradient magnetic field G is increased, the inhomogeneity of magnetic field becomes larger and hence the decrease time constant T.sub.2 * becomes smaller, thereby deteriorating S/N.
Further, since the detected signal decreases exponentially, increase of the measuring time T also results in the deterioration of S/N.
Thus, the more that G and T increase, there results more deterioration of S/N.